Artist's impression of HD265435 in 30 million years. (University of Warwick/Mark Garlick)

Two Stars Spiraling Towards Explosive Doom Detected in Our Cosmic Neighborhood

12 JULY 2021

A binary star just 1,500 light-years away is spiraling towards a spectacular doom.

HD265435 consists of a type of dead star called a white dwarf and its binary companion; they're orbiting each other so close together, the white dwarf is slurping material from the other star. Eventually, so the theory goes, the white dwarf will gain so much mass that it is no longer stable, exploding in a tremendous supernova.


That won't be for a while yet, but the discovery of such a doomed binary is a rare one, say a team of scientists led by astronomer Ingrid Pelisoli from the University of Warwick in the UK; the finding can help us better understand the processes leading up to these incredible events.

This is important, because the type of supernova this unstable star will cause is what we call a standard candle - one of the key tools we use to measure cosmic distances.

Stars spend their lives (what we call the main sequence) busily fusing elements to heavier elements in their cores, but they don't have an endless supply. Eventually, they will run out of stuff they can fuse, and die, ejecting their outer material. Depending on the mass of the star, several things can happen at this point.

For most stars, the core will collapse into an ultradense object, and what that object is will depend on the mass of the progenitor main-sequence star. For stars over 30 times the mass of the Sun, that will be a black hole. For stars between about 8 and 30 solar masses, it will be a neutron star. And for stars below 8 solar masses (including our Sun), it will be a white dwarf.


These stars still shine with residual heat, and take a very, very long time to cool to darkness. The only thing that keeps them from collapsing entirely under their own gravity is electron degeneracy pressure. At a certain pressure level, electrons are stripped from their atomic nuclei. Because identical electrons can't occupy the same space, these electrons supply the outward pressure that keeps the star intact.

That has a limit, too. Over about 1.4 times the mass of the Sun, or the Chandrasekhar limit, the white dwarf becomes so unstable that it dies again, exploding in a Type Ia supernova. This can happen when the white dwarf orbits so close to a binary companion that it siphons material from the other star, tipping it over the Chandrasekhar limit.

But there's a curious discrepancy in the number of observed Type Ia supernova remnants, and the number of Type Ia progenitor candidates - we simply haven't found as many progenitors as there should be, based on the number of observable remnants.

This is why HD265435 is so exciting. At its 1,500 light-year distance, it's the closest known Type Ia progenitor, which means we have the opportunity to study it in detail.


"We can estimate how many supernovae are going to be in our galaxy through observing many galaxies, or through what we know from stellar evolution, and this number is consistent," Pelisoli said.

"But if we look for objects that can become supernovae, we don't have enough. This discovery was very useful to put an estimate of what a hot subdwarf and white dwarf binaries can contribute. It still doesn't seem to be a lot, none of the channels we observed seems to be enough."

The binary itself is fascinating. It consists of the white dwarf, and a hot subdwarf, the latter being a red giant after it has ejected its outer layers and is about to begin fusing helium, having run out of hydrogen. This hot subdwarf is small, just 0.6 times the mass of the Sun, but extremely bright - so bright that it completely outshines the white dwarf. We can't see the white dwarf at all.

Pelisoli and her team identified the binary by changes in brightness in the hot subdwarf. These changes suggested that the hot subdwarf is being pulled into a teardrop shape by something massive very close to it.

By carefully analyzing the brightness changes, the researchers were able to infer what is happening. A white dwarf about the same mass as the Sun is orbiting the hot subdwarf every 100 minutes or so, close enough to be siphoning material from the subdwarf and pulling its atmosphere out of shape.

Together, the masses of the two objects exceed the Chandrasekhar limit, which means a Type Ia supernova should occur... in about 70 million years or so. Before that happens, the white subdwarf will run out of material to fuse and turn into a second white dwarf star.

This discovery could help us to understand a massive problem with cosmology. Because the Chandrasekhar mass is within a known range, Type Ia supernovae have a determinable intrinsic brightness. This means we can use them to map distances in the local Universe - but we use several methods to do this, and different methods give us different results for the expansion rate of the Universe.

"The more we understand how supernovae work, the better we can calibrate our standard candles. This is very important at the moment because there's a discrepancy between what we get from this kind of standard candle, and what we get through other methods," Pelisoli said.

"The more we understand about how supernovae form, the better we can understand whether this discrepancy we are seeing is because of new physics that we're unaware of and not taking into account, or simply because we're underestimating the uncertainties in those distances."

The research has been published in Nature Astronomy.